1. Field of the Invention
The present invention relates to a power transmission system for an automotive vehicle, and more specifically to a power transmission system suitable for use in an automotive vehicle in which an engine and a transmission are connected in series and arranged in the vehicle transversal direction.
2. Description of the Prior Art
A power transmission system for a 4-wheel drive vehicle in which an engine and a transmission are connected in series and arranged in the vehicle transversal direction is disclosed in Japanese Published Examined (Koho) Patent Appli. No. 62-9060, for instance, as shown in FIG. 1. This power transmission system comprises an engine 201, a transmission 203 driven by the engine 201, a center differential gear 205 connected to the transmission 203, a front differential gear 207 arranged coaxially with the center differential gear 205, and a change-direction gear assembly 209.
The center differential gear 205 distributes drive power supplied from the engine 201 through the transmission 203 differentially to the front differential gear 207 and the change-direction gear assembly 209. The front differential gear 207 further distributes engine power differentially to front left and right wheels 211 and 213, respectively. Further, a rear differential gear 219 distributes engine power transmitted through the power assembly 209 differentially to rear left and right wheels 215 and 217, respectively. As described above, the vehicle can be smoothly driven in 4-wheel differential drive mode.
In the above-mentioned power transmission system for a 4-wheel drive vehicle, however, when one of the front or rear wheels 211 and 213 or 215 and 217 slips on a slippery road, since engine power tends to concentrate to only the slipping wheel and therefore the vehicle will not be driven, two viscous coupling devices for limiting the differential motions of the center and front differential gears 205 and 207 are provided separately. However, where the viscous coupling device is simply attached to each of the center and front differential gears 205 and 207 respectively, since the width of the differential gear provided with a viscous coupling device increases in the vehicle transversal direction, there arise various problems as follows:
The lengths of the front left and right wheel shafts 221 and 223 decrease; spaces near the two front wheels 211 and 213 decrease; and therefore joint angles of joints 225 and 227 disposed between the front wheel shaft 221 or 223 and the wheel 211 or 213 increase.
In addition, since engine power transmitted to the center differential gear 205 is larger than that transmitted to the front differential gear 207 (because a four front and rear wheel driving power is supplied to the gear 205 but only a two front wheel driving power is supplied to the gear 207), the size of the viscous coupling device attached to the center differential gear 205 inveritably increases as compared with the viscous coupling device attached to the front differential gear 207. Therefore, the space near the wheel shaft 211 on the left wheel side in FIG. 1 decreases markedly as compared with that on the right wheel side, because of asymmetical device arrangement relationship with respect to the vehicle's longitudinal central line.